Boris Johnson has launched a fresh onslaught on Jeremy Corbyn warning he could not be trusted to lead the Brexit negotiations if he gained power in the General Election on June 8.

As the latest clutch of opinion polls showed the Conservatives' lead over Labour narrowing, the Foreign Secretary said Labour had no clear position on Britain's future relationship with the EU.

"We are at a critical phase in the history of this country. We have to get Brexit right. I am genuinely alarmed by the idea that it could be handled in just 11 days after the election by Jeremy Corbyn," he told ITV's Peston on Sunday.

"I do not for the life of me understand how he is supposed to go and sit at that table in Brussels on day one of the talks when he hasn't got a clue whether he wants to stay in the single market or the customs union and he has a completely unintelligible position on immigration.

"They are going to look at him and have him for breakfast. It think it will be deeply damaging to the interests of this country."

Mr Johnson also defended controversial plans set out by Theresa May in the Conservative manifesto to overhaul the system of funding social care, saying the Prime Minister had shown "bravery" in addressing a crucial issue.

"This is necessary. We face another two million over the age of 75 in the next ten years. We have to do something about the huge costs of social care," he said.

"I think it is a mark of Theresa May's bravery and candour with electorate that she is doing this. It shows the strength and purpose she will bring to everything she does if we are re-elected."

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Under the Conservative blueprint, the planned £72,000 cap on care costs would be scrapped. Instead people would not have to pay for their care if they have assets of below £100,000 - including the value of their home - rather than the current threshold of £23,500.

While payment would be able to be deferred until after a person has died, those receiving care in their own homes will for the first time be liable to pay and not just those in residential care..

Mr Johnson acknowledged that there were concerns about the plan but insisted that the "broad thrust" was right.

"Whatever happens, people are going to be able to live in their own home. They will have that anxiety taken away and they will be able to pass on a minimum of £100,000 to their kids," he said.

"I do understand people's reservations and the questions people are asking about some of the detail of this but the broad thrust right. The broad thrust has got to be brave, to be resolute and take on the problem."

Mr Johnson refused to be drawn on whether the Cabinet had been consulted in the proposals.

"There were all sorts of consultations about the manifesto. You wouldn't expect me to go into detail," he said.

Mr Johnson defended Mrs May's decision to retain the target of reducing net migration to below 100,000-a-year - even though the Conservatives in government consistently failed to meet it - saying it represented a "sensible" level of immigration, although he would not say when it could be met.

"I am not going to make any predictions of when we are going to achieve it," he said.